Stress relieving member for encapsulated transformer windings

ABSTRACT

One or more layers of foil are positioned between the start conductor-turn of the outer low-voltage winding section and the encapsulating material. The foil prevents adherence of the encapsulating material to the start conductor-turn of the outer low-voltage winding section. This permits relative movement between the outer low-voltage winding section and the encapsulating material, thus preventing the formation of mechanical stresses. The foil layers are electrically connected to the start conductor-turn of the outer low-voltage winding section to eliminate corona discharges. The ends of the foil layers are electrically insulated from each other to prevent the formation of a shorted turn within the winding assembly.

1 1" 3,750,071 [451 July 31,1973

1 1 STRESS RELIEVING MEMBER FOR ENCAPSULATED TRANSFORMER WINDINGS [75] Inventor: Edgar R. Eley, Athens, Ga.

[73] Assigneez. Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: May 5, 1972 [21] Appl. No.: 250,552

[52] US. Cl 336/84, 336/96, 336/183,

3,548,355 2,731,589 H1956 Marsh 336/84 X Primary Examiner-Th0mas J. Kozrna Attorney-A. T. Stratton, J R. Nanway et a1.

[5 7 ABSTRACT One or more layers of foil are positioned between the start conductor-turn of the outer low-voltage winding section and the encapsulating material. The foil pre vents adherence of the encapsulating material to the start conductor-turn of the outer low-voltage winding section. This permits relative movement between the outer low-voltage winding section and the encapsulating material, thus preventing the formation of mechanical stresses. The foil layers are electrically connected to the start conductor-tum of the outer low-voltage winding section to eliminate corona discharges. The ends of the foil layers are electrically insulated from each other to prevent the formation of a shorted turn within the winding assembly.

12 Claims, 12 Drawing Figures 12/1970 Martincic et al. 336/84 X STRESS RELIEVING MEMBER FOR ENCAP SULATED TRANSFORMER WINDINGS BACKGROUND OF THE INVENTION able insulating material, such as an epoxy resin, are frequen tly used in power distribution transformers. One form of encapsulated winding assembly includes an inner low-voltage winding section, an outer low-voltage winding section, and a high-voltage winding section which is positioned between the inner and outer lowvoltage winding sections. In manyencapsulated winding assemblies, the winding sections are wound with sheet conductors, such as aluminum foil. However, winding assemblies for particular ratings may be constructed from strap conductors or a combination of strap and sheet conductors. I

The encapsulated winding assembly is constructed by first winding the inner low-voltage winding section around a suitably shaped form. The high-voltage winding section is then wound around the inner low-voltage winding section with sufficient insulating material placed therebetween to provide the required electrical insulation. The outer low-voltage winding section is wound on a suitable form, removed from that form, and then placed over the high-voltage winding sectionwith a space therebetween. The winding sections are encapsulated with an epoxy resin which fills the spaces between the winding sections.

Due to different expansion characteristics, it has been found that the encapsulating materialof the winding assembly has a tendency to crack in the region which is adjacent to the sheet conductors of the winding sections. This has been especially noticeable adjacent to an outermost turn of the high-voltage winding section.

Stress relieving agents have been used in order to relieve the stresses in the winding assembly. One form of stress relieving agent used in the prior art consists of the application of a material to the winding section conductor surfaces to prevent adhesion of theepoxy to the conductor. This forms a release film which allows the conductor to slip with respect to the epoxy material adjacent to it. Silicone grease has been used successsusceptible to corona discharges.

SUMMARY OF THE INVENTION There is disclosed herein new and useful stress relieving members for encapsulated winding assemblies. The stress relieving member is positioned adjacent to the start conductor-turn of the outer low-voltage winding section of the winding assembly. The stress relieving with a part thereof broken away member is constructed of one or'more layers of a sheet material, such asaluminum foil, and is electrically connected to the start conductor-turn of the outer lowvoltage winding section. The stress-relieving member prevents the bonding of the encapsulating material to the outer low-voltage winding section and thereby allows relative movement between the winding section and the encapsulatingimaterial. There are disclosed several different arrangements for constructing the stress relieving member from a sheet of foil by folding, applying adhesive, and/or cutting slots in the foil.

BRIEF DESCRIPTION OF THE DRAWINGS sembly constructed according to the teachings of this invention;

FIG. 2 is a view of an encapsulated winding assembly to illustrate the stress relieving member; V

FIG. 3 is a plan view, partly insection, of an encapsulated winding assembly constructed according to one embodiment of this invention;

invention; and

FIGS. .5, 7, 9 and 11 are views of stress relieving members constructed by foldingthe sheet materials shown in FIGS. 4, 6,8 and 10, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description, similar reference characters refer to similar members in all figures of the drawings.

Referring now to the drawings, and FIG. 1 in particular, there is shown a power distribution transformer constructed according to an embodiment of this invention. Laminated magnetic cores 1'0 and 12 are positioned substantially adjacent to each other and may be of the wound type with the laminations first being wound into a circular shape andthen pressed into a substantially rectangular. shape. The winding assembly 14 includes an inner low-voltage winding section 16, an outer low-voltage winding section 18, and a highvoltage winding section 20. In the particular embodiment shown, all of the winding sections are constructed from sheet conductors and the high-voltagewinding section 20 includes the high-voltage winding subsections 22, 24, 26 and 28 which are interconnected by means which is not illustrated.

An encapsulating material 30 surrounds the high voltage and the low-voltage winding sections. The encapsulating material 30 may be a resin polymer such as an anhydride hardened, bisphenol-A, epoxy resin. A stress relieving'member 32 is positioned adjacent to an outside conductor-turn of the low-voltage winding section 18 and, more specifically, adjacent to the start conductor-turn thereof. The stress relieving member 32 prevents the transfer of stresses through the encap- 32, some of which will be described in more detail later, the stress relieving member 32 comprises one or two layers of a sheet material, such as aluminum foil, having an axial height substantially equal to the axial height of the outer iow-voltage winding section 18.

FIG. 2 shows, in the cut-away portion of the figure, the relative position of the stress relieving member 32. The high-voltage winding terminals 34 and 36 are connected to the high-voltage winding section 20. The lowvoltage winding terminals 38 and 40 are connected to the inner low-voltage winding section 16 and the terminal 42 is connected to the start conductor-turn of the outer low-voltage winding section 18. The other neces sary terminal which must be connected to the outer low-voltage winding section is not illustrated. Terminal 42 electrically connects one end of the stress relieving member 32 to the start conductor-turn of the outer low-voltage winding section 18. The other end of the stress relieving member 32 is positioned near its first end to provide the maximum amount of stress relief. However, the ends of the stress relieving member 32 do not electrically contact each other since this would form a shorted turn in the winding assembly 14.

Another view of the winding assembly 14, with the stress relieving member 32 shown in greater detail, is illustrated in FIG. 3. The stress relieving member 32 is positioned between the outer low-voltage winding section 18 and the encapsulating material 30. The first end 44 of the stress relieving member 32 is connected to the terminal 42. The stress relieving member 32 extends along the outer low-voltage winding section 18 with the second end 46 of the stress relieving member 32 separated from the first end 44 by the encapsulating material 30. It is within the contemplation of this invention that the first and second ends of the stress relieving member, 44 and 46 respectively, may overlap one another with a suitable insulating material placed therebetween.

The enlarged portion of FIG. 3 shows the connecting arrangement for the stress relieving member 32 in more detail. In this embodiment of the invention, the stress relieving member 32 comprises the foil layers 50 and 52 which are crimped together, by the terminal 42, with the start conductor-turn 48 of the outer lowvoltage winding section 18. The side 54 of the foil layer 52 adheres to the encapsulating material 30. The side 56 of the foil layer 50 adheres to the start conductorturn 48 which may be coated with an adhesive material, such as a B-stage thermosetting resin. The contact area 58 of the foils 50 and 52 forms a surface which allows the foils 50 and 52 to slip with respect to eachother, thus a relatively small amount of stress is transferred from the encapsulating material 30 to the low-voltate winding section 18. Since the foil layers 50 and 52 are at the same potential, electrical stresses do not exist between the'foil layers 50 and 52 and corona discharges do not present a problem.

If the conductor of the outer low-voltage winding section 18 is not coated with an adhesive, the embodiment shown in FIG. 3A may be used. FIG. 3A corresponds to the same area of the winding assembly 14 which is enlarged in FIG. 3. The conductor-turns of the outer low-voltage winding section 18, including its start conductor-turn 60, are insulated by interleaved paper insulation, a coating of fully cured enamel, or any other type of insulation which will not adhere to the stress relieving member 32. The stress relieving member 32 comprises the single foil layer 62 which is crimped to the start conductor-turn 60 by the terminal 42. The

contact area 64 between the foil layer 62 and the start conductor-turn 60 is not electrically stressed, therefore, corona in this region is not a problem. Furthermore, the foil layer 62 is free to slip with respect to the start conductor-turn 60 thereby eliminating mechanical stresses.

The foil layer, or layers, may be constructed by several different techniques. FIGS. 4 through 11 illustrate some arrangements which may be used in constructing the stress relieving member 32. However, other arrangements may be used without departing from the scope of this invention.

FIG. 4 illustrates a sheet of foil 66 which may be used in constructing a stress relieving member 32 having two foil layers. A suitable adhesive 68 is applied at various positions to the foil 66. The foil 66 is then folded about the axis 70. The resulting stress relieving member 32 is illustrated in FIG. 5 and has length and height dimensions which are substantially the same as the dimensions of the start conductor-turn 48 of the outer lowvoltage winding section 18. The spaces 72 allow gases to escape from between the foil layers of the stress relieving member 32 when under a vacuum during the encapsulating process.

FIG. 6 illustrates a sheet of foil 74 which may be used in constructing a stress relieving member 32 having two foil layers. An adhesive 76 is applied along the edge of the foil 74 as illustrated. The foil is then folded about the axis 78. FIG. 7 shows the completed stress relieving member 32 with the spaces 80 permitting the escape of gases during encapsulation.

FIG. 8 illustrates another arrangement whereby slots 82 are cut in a sheet of foil 84 along the axes 86 and 88 of the foil 84. FIG. 9 illustrates the stress relieving member 32 after being folded about the axes 86 and 88. Although the two-layer stress relieving member 32 shown in FIG. 9 can be conveniently encapsulated in the winding assembly 14 without the layers having an adhesive therebetween, it is within the contemplation of this invention that the layers may be bonded together by an adhesive material.

FIG. 10 illustrates a sheet of material 90 which may be used to construct a stress relieving member 32 suitable for use adjacent to a start conductor-turn which does not have adhesive applied thereto. An adhesive material 92 is applied to the edges of the foil 90 which has slots 94 cut therein. The foil 90 is folded about the axes 96 and 98 to produce the stress relieving member 32 illustrated in FIG. 11. The adhesive material 92 is exposed after folding and adheres to the bare start conductor-tum. I

There has been disclosed various embodiments of a stress relieving member for reducing mechanical stresses in an encapsulated transformer winding assembly without producing a region susceptible to corona discharges. Since numerous changes may be made in the above described apparatus and different embodifrom the spirit thereof, it is intended that all of the mat-' ter contained in the foregoing description, or shown in the accompanyingdrawings, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. A winding assembly for electrical inductive apparatus, said winding assembly comprising high-voltage and low-voltage winding sections encapsulated in a resinous insulating material, each of said winding sections comprising a plurality of concentric turns of electrical conductor, a stress relieving member positioned between a first of said winding sections and the encapsulating resinous insulating material to permit relative movement therebetween, said stress relieving member comprising a first layer of electrical conducting foil disposed in contact with an outer turn of said first winding section, there being-substantially no encapsulating material between said outer turn and said steess relieving member, said first layer of electrical conducting foil of said stress relieving member having first and second ends with said first end physically and electrically connected to said outer turn of said first winding, said first layer of foil having a height'dimension substantially equal to the height dimension of said first winding sec tion, and said first layer of foil extending substantially along the entire circumference dimension of said first winding section.

2. The winding assembly of claim 1 wherein the first and second ends of the first layer of foil are eletrically insulated from each other.

3. The winding assembly of claim 2 wherein the second end of the first layer of foil is located at a position along the circumference of the first winding section that provides a gap between the first andsec on d ends of the first layer of foil.

4. The winding assembly of claim 1 wherein the first end of the first layer of foil is electrically connected to the outer turn of the first winding section by a terminal member which crimps the outer turn and the first layer of foil together.

5. The winding assembly of claim 1 wherein the first winding section is wound with sheet type conducting materials.

6. The winding assembly of claim 1 wherein the stress relieving member comprises a second layer of electrical conducting foil disposed between the first layer of foil and the resinous insulating material, said second layer of foil having first and second ends with said first end electrically connected to the first end of the first layer of foil, said second layer of foil having a height dimension substantially equal to the height dimension of the first layer of foil, the second end of said second layer of foil being positioned substantially adjacent to the second end of the first layer of foil with an adhesive material disposed between the first layer bf foil and the outer turn of the first winding section.

7. The winding assembly of claim 6 wherein the first and second layers of foil comprise asingle foil sheet having a height dimension substantially equal to the height dimension of the first layer of foil, said single foil sheet being folded about an axis parallel to the height dimension of said single foil sheet.

8. The winding assembly of claim 6 wherein the first and second layers of foil comprise a single foil sheet having a height dimension substantially equal to twice the height dimension of the first layer of foil, said single foil sheet'being folded about an axis perpendicular to the height dimension of said single foil sheet.

9. The winding assembly of claim 6 wherein the first and second layers of foil comprise a single foil sheet having a height dimension substantially equal to twice the height dimension of the first layer of foil, said single foil sheet being folded about two axes which are perpendicular to the height dimension of said single folded sheet. v I

10. The winding assembly of claim 6 wherein adhesivematerial is placed between the first and second layers of foil with openings between adhesive locations to provide means for gases to escape from between the layers of foil.

1 l. The winding assembly of claim 6 wherein the layers of foil include slots which provide means'for gases to escape from between the layers of foil.

l2. Anencapsulated transformer winding assembly comprising an inner low-voltage winding section, an outer low-voltage winding section, a high-voltage winding section disposed between said inner and outer lowvoltage winding sections, said outer low-voltage wind ing section being wound with an electrical conducting sheet material to form start and finish conductor-turns, said transformer winding assemly being encapsulated with a resinous material, a stress relieving memberpositioned in contact with the start conductor-turn of said outer low-voltage winding section and the encapsulating material, said stress relieving member permitting relative movement between said encapsulating material and said outer low-voltage winding section, there being substantially no encapsulating material between said start conductor turn and said stress relieving member, said stress relieving member comprising a layer of electrical conducting sheet material having first and second ends, said first end of said layer being physically and electrically connected to the start conductor-turn by a terminal member, and said second end of said layer being positioned to provide a gap between the first and second ends of said sheet material.

* I! I i 

1. A winding assembly for electrical inductive apparatus, said winding assembly comprising high-voltage and low-voltage winding sections encapsulated in a resinous insulating material, each of said winding sections comprising a plurality of concentric turns of electrical conductor, a stress relieving member positioned between a first of said winding sections and the encapsulating resinous insulating material to permit relative movement therebetween, said stress relieving member comprising a first layer of electrical conducting foil disposed in contact with an outer turn of said first winding section, there being substantially no encapsulating material between said outer turn and said steess relieving member, said first layer of electrical conducting foil of said stress relieving member having first and second ends with said first end physically and electrically connected to said outer turn of said first winding, said first layer of foil having a height dimension substantially equal to the height dimension of said first winding sectIon, and said first layer of foil extending substantially along the entire circumference dimension of said first winding section.
 2. The winding assembly of claim 1 wherein the first and second ends of the first layer of foil are eletrically insulated from each other.
 3. The winding assembly of claim 2 wherein the second end of the first layer of foil is located at a position along the circumference of the first winding section that provides a gap between the first and second ends of the first layer of foil.
 4. The winding assembly of claim 1 wherein the first end of the first layer of foil is electrically connected to the outer turn of the first winding section by a terminal member which crimps the outer turn and the first layer of foil together.
 5. The winding assembly of claim 1 wherein the first winding section is wound with sheet type conducting materials.
 6. The winding assembly of claim 1 wherein the stress relieving member comprises a second layer of electrical conducting foil disposed between the first layer of foil and the resinous insulating material, said second layer of foil having first and second ends with said first end electrically connected to the first end of the first layer of foil, said second layer of foil having a height dimension substantially equal to the height dimension of the first layer of foil, the second end of said second layer of foil being positioned substantially adjacent to the second end of the first layer of foil with an adhesive material disposed between the first layer of foil and the outer turn of the first winding section.
 7. The winding assembly of claim 6 wherein the first and second layers of foil comprise a single foil sheet having a height dimension substantially equal to the height dimension of the first layer of foil, said single foil sheet being folded about an axis parallel to the height dimension of said single foil sheet.
 8. The winding assembly of claim 6 wherein the first and second layers of foil comprise a single foil sheet having a height dimension substantially equal to twice the height dimension of the first layer of foil, said single foil sheet being folded about an axis perpendicular to the height dimension of said single foil sheet.
 9. The winding assembly of claim 6 wherein the first and second layers of foil comprise a single foil sheet having a height dimension substantially equal to twice the height dimension of the first layer of foil, said single foil sheet being folded about two axes which are perpendicular to the height dimension of said single folded sheet.
 10. The winding assembly of claim 6 wherein adhesive material is placed between the first and second layers of foil with openings between adhesive locations to provide means for gases to escape from between the layers of foil.
 11. The winding assembly of claim 6 wherein the layers of foil include slots which provide means for gases to escape from between the layers of foil.
 12. An encapsulated transformer winding assembly comprising an inner low-voltage winding section, an outer low-voltage winding section, a high-voltage winding section disposed between said inner and outer low-voltage winding sections, said outer low-voltage winding section being wound with an electrical conducting sheet material to form start and finish conductor-turns, said transformer winding assemly being encapsulated with a resinous material, a stress relieving member positioned in contact with the start conductor-turn of said outer low-voltage winding section and the encapsulating material, said stress relieving member permitting relative movement between said encapsulating material and said outer low-voltage winding section, there being substantially no encapsulating material between said start conductor turn and said stress relieving member, said stress relieving member comprising a layer of electrical conducting sheet material having first and second ends, said first end of said layer being physically and electrically connected to the start Conductor-turn by a terminal member, and said second end of said layer being positioned to provide a gap between the first and second ends of said sheet material. 